U.S. Pat. No. 5,923,164 describes a signal tuning apparatus including a gain element, a variable impedance and a control device. The gain element is an operational amplifier having an input for receiving an input signal to be tuned, and an output with an output signal. A feedback path extends from the output to the input. The variable impedance is provided in the feedback path for varying the gain of the operational amplifier. The variable impedance includes a plurality of resistors and a plurality of switches. The resistors are connected in series in the feedback path. Each of the resistors has a switch in parallel for switching the resistor into or out of the feedback path. The output signal of the operational amplifier is connected to the control device as an input signal. Based on this input signal, the control device generates a plurality of switching signals which are provided to the switches of the variable impedance. Furthermore, the control device receives a minimum reference voltage and a maximum reference voltage which represent a bandwidth for the output signal of the operational amplifier.
In operation, the signal tuning apparatus of U.S. Pat. No. 5,923,164 compares the output signal of the operational amplifier with the minimum and maximum reference voltage. If the output signal is smaller than the minimum reference voltage or greater than the maximum reference voltage, the gain of the operational amplifier is increased or decreased by switching appropriate resistors into or out of the feedback path of the operational amplifier. If the output signal is greater than the minimum reference voltage and smaller than the maximum reference voltage, then the gain of the operational amplifier is not changed. Thus, the output signal of the operational amplifier is held within the bandwidth defined by the minimum and maximum reference voltage.